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Reconstructing a 15th century German frame saw: Developing a prototype

I often find myself making my own tools, so that I can make more authentic products, because completing a medieval woodworking project with modern tools versus period hand tools often changes not only the way I work, but also the look or even the shape of the item produced. I still find myself using modern power tools frequently, and this project has been no different. I used a scroll saw, an electric lathe, and a drill press, as well as some period hand tools, such as a block plane, a flush cut saw, and assorted chisels and files. The idea for this project arose from my need to be able to make a straight cuts in raw material wood, in order to produce custom size planks for making chests and other boxes, as well as blocks for turning. The tool medieval woodworkers used for this was a frame saw, the precursor to the modern bow saw. It functions somewhat like a coping saw, in that the blade may be rotated within the frame, but with a broader blade, so it is better suited for long straight cuts than the curving cuts for which the coping saw is known.

This saw, illustrated in folio 21r of the Hausbucher of the Mendelschen Zwolfbruderstiftung (circa 1414), features a three-piece frame shaped like a big H, with a pair of handles on both ends of the saw blade and a string and wedge to maintain tension on the opposite side of the frame. Several minor variations are possible within this general description, and several published sources include illustrations of varying quality, but I have not yet found a useful extant specimen upon which to base specific measurements or to get other information such as which species of wood was used. Beech was sometimes used for tool handles, but ash and oak may also be suitable materials, and it is certainly possible that the species used, as well as the quality of the product, may have varied as widely then as the tools of today. I made my prototype from a pine 2×4 (actual size is 1-1/2 inch by 3-1/2 inch) because it was cheap scrap that I had on hand in my shop. I doubt pine would have been used by a medieval tool maker, and my plan from the beginning was to use North American white oak as a substitute for authentic European oak, but I wanted to work out a few details in actual wood before using valuable oak or ash, so a prototype built from scrap pine is a good idea. After building my prototype, I am glad I did this extra step. As it turns out, I did learn a few things in the process that I would have otherwise overlooked.

Outlining the main pieces of the saw

My first task was to work out the appropriate length of the three main pieces and allow for the two turned knobs. I then cut these out on a scroll saw. A medieval toolmaker would have used a frame saw like this one to make the straight cuts (I guess you had to have a saw to make a saw) and a coping saw to make the curved cuts. Next, I turned the knobs on a lathe, checking that the shanks on both were uniform in diameter. Using a wood vice to hold each handle steady, I cut a slit through the center of each shank, and then used a drill press to drill a small hole through each, about 1/2 inch down from the tip, to accommodate the pins which will hold the saw blade in place.

Cutting the slit in the handle, to accommodate the end of the saw blade. The shank must be drilled perpendicular to this cut for a pin to hold the saw blade in place.

Moving on to the crossmember, I rounded off both ends and then cut a 1/2 inch tenon, so the resulting tenons are D shapes 1/2″ thick. The D shape allows the notched uprights to rock over the tenon as tension is applied. Finally, I shaved the crossmember to about half its original thickness (about 3/4 inch). This would have been done with a draw knife, but since this is a prototype and not a masterwork, I sped up the process by going back to the scroll saw.

Cutting the notch for the rounded tenon, using a half-inch wide chisel

Next, I notched the uprights with a half-inch wide chisel to accommodate the tenons of the crossmember. Then I drilled the uprights to fit the knobs and cut a pair of pins from an old nail. After a dry fit, I found that the whole frame is a bit taller than necessary and entirely too heavy to allow ease of use. This allowed me to make a couple decisions. For one, the uprights may be a bit more heavily built than necessary. It will be interesting to make a simple pair of bows (more like the 1414 illustration above), without heavier places, and see how a lighter weight version compares. Another decision I made was to begin with 1 inch stock instead of 1-1/2 inch stock, resulting in a lighter frame, but this will also require me to make the knobs a bit smaller as well, with a longer and narrower shank. This means I will need to add 3/4 inch of length to the shanks of both knobs and 1-1/2 inch to the crossmember, so that the pins may be placed and removed without pulling the saw blade into the holes through the uprights, since these holes will need to be smaller due to the narrower stock. I also decided it may be a good idea to square the knob shanks and mortise the holes, in order to improve stability against blade twisting.

The finished prototype

My next step at this point is to make a model 2 prototype using 1 inch stock, incorporating a more simplified design of the uprights, longer shanks on the knobs, a longer crossmember, and slightly shorter uprights. My goal is to make the model 2 prototype weigh about half as much as this one.

UPDATE 2014-09-23:

After completing my second prototype, I have to say I am much happier with it than I was with the one outlined above. The thing I realized when I went back to the 1414 sketch was that in designing my first prototype I was thinking too much like a modern woodworker, designing complex bows that were sleek in some places and more sturdily built in other places for added strength at key locations. This produced a saw frame that was too bulky and too heavy. For my second prototype I went back to the 1414 sketch and copied it as closely as possible. One thing I learned was that the simple curves of these bows allow for four of them (enough to make two saws) to be cut from a single board and produce almost no waste wood. I am very pleased with my prototype and do not wish to make any significant changes to the design, and I think it looks more delightfully medieval than my clunky prototype in the photos above.

Old and new frame saw prototypes. The old prototype (background) is cut from a pine 2×4, and the newer one (foreground) is cut from 4/4″ white oak. This lighter and more graceful design will be my go to for period frame saws.